Flying spot scanners, often referred to as raster output scanners (ROS), conventionally have a reflective multi-faceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (also known as the fast-scan direction) while the recording medium is being advanced in an orthogonal, or process, direction (also known as the slow-scan direction) such that the beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample string, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. Printers that sweep several beams simultaneously are referred to as multi-beam printers. Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura, the disclosure of which is incorporated herein by reference.
In the Kitamura patent, multiple lasers are arranged diagonally (see FIG. 10b of the Kitamura patent) to sweep multiple beams across a single photoreceptor. The beams are also displaced from each other in the cross-scan direction so that multiple lines can be scanned simultaneously across the photoreceptor. An object of the Kitamura patent is to reduce variations in pitch by spacing individual lasers within the laser array closely in a compact structure.
High speed process color and multi-highlight color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate locations. This is called multi-station printing.
Conventional architectures from multi-station process color printers use a plurality of separate ROSs, usually four independent ROSs, as illustrated in U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al., the disclosures of which are incorporated herein by reference. Problems with these systems include the high cost of providing multiple ROSs, the high cost of producing nearly identical multiple ROSs and the difficulty of registering system colors.
U.S. Pat. No. 5,243,359 to Fisli, the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. In the Fisli patent, the rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having their largest divergence angles parallel to one another, that are subsequently separated by a plurality of optical filters and directed onto their associated photoreceptors. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit. The laser diodes are arranged in a line in a cross-scan direction, i.e., parallel to the axis of rotation of the polygon mirror.
Commonly assigned U.S. patent application Ser. No. 07/948,531, to Thomas L. Paoli, the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS system in which the rotating polygon mirror simultaneously deflects a plurality of orthogonally polarized and dissimilar wavelength laser beams having their largest divergence angles parallel to one another. The deflected laser beams are subsequently separated by a polarized beam separator and by a plurality of dichroic beam separators and directed onto their associated photoreceptors. Similarly, the dimensions of the spots formed on each photoreceptor are controlled by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit. The laser diodes are arranged in a line in a cross-scan direction and must be fabricated such that they are packed closely together in a direction parallel to the polygon mirror rotation axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. That is, the laser diodes are kept as close together as possible in the cross-scan direction so that the light beams strike as nearly the same portion of the polygon mirror as possible.
Commonly assigned U.S. Pat. No. 5,341,158 entitled "A Raster Output Scanner for a Multi-Station Xerographic Printing System Having Laser Diodes Arranged in a Line Parallel to the Fast Scan Direction", to James J. Appel et al., the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS architecture in which the laser diodes are positioned along a line that is parallel to the fast scan direction of the ROS (perpendicular to the rotation axis of the polygon mirror) and are tangentially offset in the fast scan direction.
Commonly assigned, U.S. patent application Ser. No. 08/156,219 entitled "Offset Mounting of Nonmonolithic Multiwavelength Lasers" to Kovacs et al., the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS architecture in which the laser diodes, which produce laser beams of different wavelengths, are axially displaced from one another. The laser producing the beam having the shortest wavelength is located closest to an F.theta. scan lens and the laser producing the beam having the longest wavelength is located farthest from the F.theta. lens. Because the focal length of the F.theta. lens is dependent on the wavelength of the transmitted laser beam, axially offsetting the laser sources results in the laser beams being focused is substantially the same plane. In U.S. patent application Ser. No. 08/156,219, as well as the above-referenced patents and patent applications that direct multiple beams separated by wavelength to separate photoreceptors, the beams are separated by wavelength or wavelength and polarization by beam separators. However, none of the above-referenced patents or patent applications recognize or solve the problem that the optics and optical separators through which the beams pass have a different index of refraction for beams having dissimilar wavelengths. This results in the images being fixed on the separate photoreceptors having differing image heights in the tangential direction. This problem causes improper registration of the images on the imaging medium.